A preparation of single cardiac cells from which the surface membrane has been removed by microdissection (skinned cardiac cells) has been developed by the applicants. The preparation permits the perfused solution to come in direct contact with the structures that participate in the contraction and the excitation-contraction coupling of cardiac muscle. Previous work has indicated that the contractions of this preparation are activated by a Ca2 ion-induced release of Ca2 ion from the sarcoplasmic reticulum. During the second year of this grant, the properties of the myofilaments will be explored by direct tension recording with a transducer sensitive in the microgram range and by high speed cinematography of the sarcomere during contraction. It is proposed: 1. To study reasons for which resting tension is much higher at a give sarcomere length in the cardiac cell as compared to the skeletal muscle cell; 2. To define the sarcomere length-tension diagram of completely and partially activated myofilaments. These two studies are important to the understanding of the cellular mechanism of Starling's Law of the heart. 3. To study the effect of variations of ph on the CA2 ion sensitivity of the myofilaments. The last study is important to the understanding of the contractile effect of cardiac ischemia and infarction since it is believed that an intracellular acidosis occurs under these conditions. The effects of cell length and of variation of ph on the function of the sarcoplasmic reticulum will also be studied with respect to both the Ca2 ion release that causes activation and the CA2 ion resequestration that causes relaxation. As previously, the CA2 ion motion into and out of the sarcoplasmic reticulum will be inferred from tension transients observed in the presence of low concentration Ca 2 ion buffers and induced by the various factors under study and by caffeine. BILBIOGRPAHIC REFERENCES: A. Fabiato and F. Fabiato (1977): Calcium release form the sarcoplasmic retiuculum. In press in Circ. Res. 40(2), February, 1977, about 12 pages. A. Fibiato and F. Fabiato (1977): Ca2 ion-induced release of Ca2 ion frrm the cardiac sarcoplasmic retiuculum (Abstract). In press in Proceedings of 27th International Congress of Physiological Sciences.